1. Field of the Invention
This invention relates to a heat sink apparatus for cooling a highly heat-generative semiconductor device such as a microprocessing unit (hereinafter referred to as MPU).
2. Description of the Prior Art
Heretofore, heat sink apparatus have been used for cooling highly heat-generative semiconductor devices and the like. Particularly, in order to deal with a large amount of heat generated by a MPU or the like, a fan motor integrated type heat sink apparatus has recently been used in which a heat sink has a built-in miniature fan to assure a high cooling effect.
Description will be given below of a conventional heat sink apparatus. FIG. 3 is a partially cut-away front view of a conventional heat sink apparatus, and FIG. 4 is a side view of the conventional heat sink apparatus partially shown in section taken on the line X-Y of FIG. 3.
In FIGS. 3 and 4, a motor 101 is a radial gap outer rotor type motor and an impeller 102 is a rotor having a plurality of vanes 103, the impeller 102 and the motor 101 for rotating the impeller 102 constituting a fan. A housing 104 of the motor 101 and an outer frame 106 are connected by means of arms 105. The outer frame 106 is fixed to a heat sink 107 by means of screws 110 or the like. The heat sink 107 is provided with a plurality of radiation fins 108 on a base 109 thereof. The base 109 has flat surfaces on both sides so that a heating element such as MPU can be mounted on one surface of the base 109 on which no fins 108 are provided. Rotation of the impeller 102 causes the vanes 103 to generate an air flow moving from an upper surface of the impeller 102 toward the base 109 of the heat sink 107 so as to let the air out of the heat sink 107 from the side.
With the above conventional construction, since the base of the heat sink was flat, the air flow generated by the fan could not be smoothly let out of the heat sink to cause an eddy to occur between a lower portion of the impeller and a central potion of the base to thereby increase the pressure loss of the air flowing through the fins. As a result, the cooling effect of the fan used for forcibly cooling the MPU by air was deteriorated, with the result that it was not always possible to obtain a heat sink apparatus having a sufficient cooling effect.
Further, since the base of the heat sink was flat, the air flow generated by the fan could not efficiently be let out of the heat sink to cause the eddy to occur between the lower portion of the impeller and the central portion of the base, giving rise to a problem of making a noise.
FIG. 5 shows a temperature distribution of a package of a MPU. In FIG. 5, reference numeral 205 denotes a temperature distribution curve in which the temperature difference between the central and peripheral portions of a package 202 is large because of the low thermal conductivity of the package 202 of a MPU 201. Reference numeral 203 denotes a cover of the package 202, and 204 denotes pins of the MPU. In the conventional heat sink apparatus whose height is made uniform, if the height of radiation fins is increased for the purpose of enhancing the cooling effect by heat radiation, the thickness of the base of the heat sink is made thinner to deteriorate the heat transfer efficiency, while if the thickness of the base is increased for the purpose of enhancing the cooling effect by heat transfer from the package central portion to the heat sink peripheral portion, the fins cannot be made high and hence the surface area of the fins is reduced to thereby deteriorate the cooling effect by heat radiation.